In the oil and gas industry, non-straight wells are formed using directional drilling techniques. The drilling process involves initially drilling a pilot borehole using a drill string with a drilling bottom hole assembly (BHA) having a drill bit at the leading end thereof. The pilot borehole must be subsequently reamed so as to accommodate casing, production pipe, etc., and to improve borehole gauge, shape and condition, using one or more reaming tools. The so formed well is then “completed”, i.e. made ready for production by installing casing (if applicable), production pipes and associated tools and perforating and stimulating the bottomhole.
More specifically, firstly the drill bit is advanced downward until the drill bit has reached the target depth. The drillstring is then pulled out and a reaming BHA is attached at the leading end of the drillstring. This reaming BHA is then pushed downhole, reaming and conditioning the borehole prior to well completion and subsequently pulled out while at the same time reaming the borehole on the way out. As discussed above, this type of reaming requires a specific reaming run after the pilot hole has been created and prior to performing well completion, which is inconvenient and inefficient. The necessity to pull the drilling BHA out of borehole to replace it with a reaming BHA results in expensive non-production time.
Poor borehole quality is generally observed and can result from:                deviation of the directional path of the well and the directional behavior of the steering device from the planned track creating tortuosities;        poor drilling practices causing vibrations and whirl action at the drill bit;        improper hole cleaning causing sedimentation;        improper drilling fluid composition not suppressing some aspects of the formation transience and formation swelling after being drilled;        BHA moving through different formations resulting in the drillstring straying from prescribed path;        the drilling tools not performing as they should whereby the drill bit leaves behind an improper hole quality and steering device oscillating about specified target path; or        drilling through interbedded formation resulting in presence of hard stringers.        
Poor hole quality affects the drilling and completions processes causing the following problems:                problems running in hole and pulling out of hole due to the obstacles that face the drillstring moving forward and backward;        poor dynamic response of the BHA; and        problems in running completions down and in its proper deployment.        
Accordingly, borehole conditioning is a factor in conducting efficient drilling and completions processes, especially in complex operations that range from:                multi-lateral wells;        deep horizontal wells;        extended reach drilling;        high pressure high temperature wells; and        high angled well profiles: J-profile, S-profile.        
Poor borehole conditioning results in non-productive time (NPT) in all steps of the drilling and completions processes. Thus it is a general practice to conduct a dedicated reaming run with controlled parameters after the drilling run, to try to condition the borehole to desirable quality.
Various elements have been included in drilling BHAs in order to improve borehole quality during drilling. Such elements include fixed blade reamers or roller reamers, which indeed can provide a degree of reaming during drilling.
Document EP 1 811 125 discloses a vibration damping reamer for use in association with a drill stem and a drill bit in drilling a hole in a rock formation. The vibration damping reamer comprises a body having a through going bore passage of drilling fluids there through. The body has a cylindrical upper part adapted for connection to a drill string, and a cylindrical lowermost reamer part of diameter wider than the upper part. The reamer part comprises a plurality of wear surfaces located on an outer surface and arranged along at least an upper band of wear surfaces and a lower band of wear surfaces axially spaced along the reamer part and separated by an intervening band. By this arrangement of axially spaced reaming bands along the length of the body, the reamer is brought to bear against the drilled hole-wall at axially spaced positions. The surfaces of the reamer bear against the hole-wall thereby ensuring any vibrations induced by the tool are damped. The adoption of two or more bearing points, represented by the wear surfaces provides stability in the position of the reamer and thereby of the drill assembly. Spiral flutes are cut on the outer surface and evenly spaced around the circumference of the body. Spiral flutes serves to allow for passage of drilling fluid and entrained rock material collected from a hole during a drilling operation. In order to create the desired vibration damping effect, the wear surfaces are studded with an array of hardwearing carbide studs. However this tool only has the effect of minimizing the lateral movement of the bit and hence extending the life of the bit and increasing the drilling rate of penetration, yet if the hole quality is poor due to any of the multiple reasons stated earlier this tool won't eliminate the need for utilizing a dedicated reaming run.
During drilling as well as during POOH (Pulling out of the hole), it is desirable to prevent stuck pipe occurrences and to be able to handle the repercussions of such situations such as:                Mobile formations        Fractured and faulted formations        Reactive formations        Over-Pressured formations        Hole cleaning problems        Unconsolidated formations        Key seats        Ledges and micro doglegs        Swelling shale and salt formations        Heavy back reaming        Tight spots        
Document WO2004/029402 discloses a combined reamer and stabilizer tool. This tool comprises a body on which is provided:                A stabilizer comprising a portion of larger diameter than the body and fluted by a series of helically shaped flutes forming a plurality of ribs destined to be in contact with the wall of the bore being drilled and;        A reamer portion below said stabilizer, comprising cutting means.        
Optionally, three rows of carbide inserts are provided up to the stabilizer for assisting the withdrawal of the drill string.
However the reaming structure uses conventional full dome shaped carbides or other cutting means with different materials. The disadvantage being that the gauge held by this full dome shaped carbide will change as the carbide starts to develop a wear area as it contacts the formation. This inherent disadvantage renders the tool as not holding the required gauge for long and this affects the directional behavior of the drilling operation.
In view of the above, it is an object of the present invention to obviate and mitigate drawbacks of the prior art and to provide a downhole assembly, a method and a tool suitable for hole drilling and reaming which are more convenient and efficient compared to existing reaming methods and tools and a method of manufacturing such a tool.
More particularly, it is an object of the present invention to avoid the requirement for a specific reaming run to condition the borehole after drilling a borehole and prior to a well completion run.
It is another object of the present invention to eliminate any borehole undergauge which may occur due to excessive formation swelling.
It is desirable to improve borehole shape, straightness and quality by removing ledges and micro-doglegs. It is also required to have a reaming tool that holds the gauge for the longest possible duration for better direction control. High quality of the hole is desirable to solve completions deployment problems due to poor hole quality and to increase the BHA dynamic performance.
It is desirable to prevent stuck pipe occurrences during drilling as well as during POOH (Pulling out of the hole).